The invention relates to a structure of internal combustion engine for returning oil which has been fed to a valve actuating mechanism and then received on the top surface of a cylinder head into a crankcase.
In FIG. 5 showing a conventional structure of internal combustion engine, the reference numeral 1 represents a cylinder block; 2 is a cylinder head; 3 is a rocker cover demarcating a rocker case in cooperation with the cylinder head 2; and 4 is an oil pan. The lower part of the cylinder block 1 and the oil pan 4 constitute a crankcase 5. Within the crankcase 5, there is arranged a crankshaft 7 which rotates in the direction that an arrow shows, with a connecting rod 6 pivotally secrued thereon. The connecting rod 6 is connected at its upper end with a piston 6a, and this piston 6a demarcates a combustion chamber in cooperation with a cylinder member which is composed of the cylinder block 1 and the cylinder head 2. The dotted line 0 in the crankcase 5 shows the locus of the big end of the connecting rod. Within the rocker cover 3, a valve actuating mechanism 8 is arranged which comprises a camshaft, a rocker arm and the likes, and this valve actuating mechanism 8 is powered from the crankshaft 7 to actuate the intake valve and exhaust valve for the combustion chamber. To an intake manifold 9 connected with the cylinder head 2, there is connected one end of a breather hose 10 which communicates the rocker cover 3 to the manifold 9. An oil dropping hole 11 and a blow-bye gas rising hole 12 which communicate a top surface 2a of the cylinder head 2 to the inside of the crankcase 5 are formed in the cylinder block 1 and the cylinder head 2. The cylinder head top surface 2a is adapted to receive an oil OB fed to the valve actuating mechanism 8. The upper end opening 12a of the blow-bye gas rising hole 12 is arranged at a position higher than the upper end opening 11a of the oil dropping hole 11.
When the engine is started and an oil pump (not shown) is operated, the oil OA reserved in the oil pan 4 is fed to the valve actuating mechanism 8. The oil which has been fed to the valve actuating mechanism 8 drops onto the top surface 2a of the cylinder head 2 and is received thereon, and then it is caused to flow down into the crankcase 5 through the oil dropping hole 11. On the other hand, the blow-bye gas which has leaked into the crankcase 5 rises into the rocker cover 3 through the blow-bye gas rising hole 12, and it is caused to flow into the intake manifold 9 through the breather hose 10. The blow-bye gas rising hole 12 also serves as an air inlet hole when the oil OB is returned to the crankcase 5.
After the engine is started and the oil pump (not shown) is operated, a part of the oil OA reserved in the oil pan 4 is sent to the cylinder head through an oil gallery and fed to the valve actuating mechanism 8. The oil OB discharged from the valve actuating mechanism 8 is received on the cylinder head top surface 2a and then falls spontaneously so as to return from the oil dropping hole 11 into the crankcase 5.
Within the crankcase 5, on the other hand, an air flow in the direction of an arrow A is generated with the rotation of the crankshaft 7. This air flow is blown into the opening 11b of the oil dropping hole 11 which opens into the crankcase 5, as shown by the arrows a, so that it serves as a dynamic pressure in a direction of hindering the flowing-down of oil. The oil dropping hole 11 may be formed so as to have a size enough to effect the dropping of oil regardless of the air flow a. However, it is in fact impossible to form said hole 11 in a large size because of the compactification of the engine. In addition, the quantity of the oil fed to the valve actuating mechanism 8 will be increased as the rotation number of the engine is increased, and as a result, the quantity of the oil OB reserved on the cylinder head top surface 2a will be also increased. If the oil quantity is extremely increased, the oil will block the blow-bye gas rising hole 12. The return of oil through the oil dropping hole 11 will be therefore insufficient and the oil quantity to be reserved in the oil pan 4 would be in short supply. Thus, there is such a problem that air is mixed into the oil sent out by the oil pump and the air-mixed oil is fed to a part to be lubricated.